A polymer electrolyte fuel cell has a high energy conversion efficiency, is clean, and is quiet, so there is an expectation for a future energy generating apparatus. In recent years, there are applications to automobiles, domestic power generators, and the like. Further, owing to a high energy density and a low operation temperature of the polymer electrolyte fuel cell, when the polymer electrolyte fuel cell is mounted to small electrical equipment such as a mobile phone, a notebook computer, and a digital camera, the small electrical equipment can be driven for a longer time than a case of using a conventional secondary battery. Accordingly, the polymer electrolyte fuel cell receives attention.
However, while the polymer electrolyte fuel cell can be driven at operating temperature of 100° C. or less, there is a problem in that with passage of power generation time, a voltage gradually decreases and power generation stops at last. This is due to so-called “flooding”, in which water generated by reaction is retained in a space serving as an air hole for a fuel gas, thereby inhibiting supply of the fuel gas which is a reactant and thus stopping power generation reaction. In particular, the flooding tends to occur in a catalyst layer on a cathode side, in which water generates.
Further, in order to put the fuel cell into practical use for the small electrical equipment, downsizing of a system as a whole is essential. In particular, in a case of mounting the fuel cell to the small electrical equipment, the fuel cell itself also has to be downsized, so, in many cases, there is adopted a method of supplying air from the air hole to an air electrode by natural diffusion without using a pump, a blower, or the like (air breathing method). In this case, a product water can be discharged to an outside of the fuel cell only by natural evaporation, so the product water tends to be retained in the catalyst layer thereby causing the flooding.
In general, a separator provided to a gas supply and exhaust portion of the fuel cell is provided with a fluid passing groove for preventing the flooding. The fluid passing groove is used as a gas diffusion path and a drainage path for the product water. Further, in order to smoothly perform drainage of the water, a groove surface of the separator is applied when necessary with a water repellent such as polytetrafluoroethylene (PTFE) or a separator material, a groove machining method, a groove shape, or the like is devised. Further, of the fuel cells using the air breathing method, there is one which is devised by using foamed metal or the like in place of the separator so that a space ratio of the gas supply and exhaust portion is increased to 90% or more to promote the natural diffusion of the product water or to utilize the space as the drainage path for the product water.
However, when the fuel cell is driven at high current density for a long time, voltage drop of the fuel cell is caused. This is because the water vapor generated by the power generation is condensed in holes of a gas diffusion electrode formed of a gas diffusion layer (hereinafter, also referred to as GDL) and the catalyst layer, thereby causing the flooding in the gas diffusion electrode.
Further, in a fuel cell of a type in which a membrane electrode assembly (hereinafter, also referred to as MEA) is sandwiched by separators having gas passing grooves and in a fuel cell of a stack type, depending on a position in an electrode surface, a diffusion distance of a reaction gas and water vapor from an outside of a cell unit differs. Therefore, in a plane of the gas diffusion electrode, partial pressure of water vapor distribution is caused. In this case, when the fuel cell is driven at high load for a long time, a difference in the partial pressure of water vapor distribution increases, thereby locally approaching a saturated vapor pressure. As a result, a product water vapor is locally condensed in the holes in the gas diffusion electrode to occupy the holes, thereby causing the local flooding.
In order to prevent the flooding in the gas diffusion electrode as described above, the drainage performance of the gas diffusion electrode needs to be improved.
For this purpose, in general, the inside of the holes of the GDL and the catalyst layer are often imparted water repellency by a water repellent or the like such as PTFE.
As a specific material of the GDL, there is used a carbon cloth, carbon paper, or the like including a mixture of carbon fibers each having a diameter of several micrometers and a hydrophobic resin such as PTFE.
Alternatively, a carbon cloth or carbon paper used as a base substrate and having one surface or both surfaces coated with a microporous layer or microporous layers including a mixture of carbon fine particles and the hydrophobic resin is also used as the GDL. This is because the microporous layer enables to reduce a contact resistance between the catalyst layer or a current collector and the GDL. In this specification, a term “GDL” includes a conductive porous body including the microporous layer and the base substrate.
In order to impart the water repellency to the catalyst layer, there is generally adopted a method of mixing fine particles including hydrophobic polymer such as PTFE with catalyst fine particles, carbon-carrying fine particles, or the like. Note that PTFE fine particles are non-conductors and have no catalytic activity. Accordingly, when a large amount the PTFE fine particles is added in order to improve hydrophobic property of the catalyst layer, the catalytic activity and a catalyst utilization rate are reduced.
As described above, while the GDL and the catalyst layer each have the water repellency, in many cases, the water repellencies of those are adjusted such that the water repellency of the catalyst layer is higher than that of the GDL. The adjustment is preformed in this manner in order to move condensed water from the catalyst layer to the GDL and to prevent backflow of the water from the GDL to the catalyst layer.
However, in the porous body imparted the hydrophobic property, water receives a force to be pushed to the outside from the porous body. Therefore, impregnation of the hydrophobic porous body with water involves a large resistance in theory, so an infiltration rate of the water into the GDL is low.
Therefore, when the fuel cell is driven at high load, a generation amount of the product water generated by being condensed in the catalyst layer exceeds an amount corresponding to the infiltration rate into the GDL. Accordingly, the condensed water is caused to be retained in an interface between the GDL and the catalyst layer.
In contrast, there are cases where the product water is condensed in the GDL. In this case, the product water is pushed out to the surface of the GDL, but a part of the condensed water is caused to deposit on the interface between the GDL and the catalyst layer. Both the catalyst layer and the GDL are hydrophobic, so the water cannot easily infiltrate into the holes thereof. As a result, the condensed water retained in the interface is caused.
In a case where output density of the fuel cell is low, all the product water is diffused in a form of water vapor, so the above-mentioned problem does not occur. However, when the fuel cell is driven at high current density for a long time, as described above, due to a difference in diffusion rate distribution of water vapor, the number of portions increases, where the partial pressure of water vapor locally rises so as to approach the saturated vapor pressure in the gas diffusion electrode. Accordingly, an amount of the product water condensed in the GDL or the catalyst layer increases as a driving time passes. In this case, the condensed water retained in the interface between the GDL and the catalyst layer as described above increases.
When the product water is retained in the interface between the GDL and the catalyst layer, a water layer having a large area is formed even by a small amount of water. Accordingly, an area for supplying a reactant gas to the catalyst layer is largely reduced. As a result, a voltage of the fuel cell is reduced by a large amount or the power generation is stopped.
As described above, with a method of imparting the water repellency to the GDL and the catalyst layer, there is a problem in that, in the case where the fuel cell is driven at high load for a long time, the condensed water is retained in the interface between the GDL and the catalyst layer. Accordingly, it is hard to say that the water repellency of the gas diffusion electrode is effectively improved.
In order to solve the problem mentioned above, there is devised a method in which, in the GDL, fine particles of two types which differ from each other in hydrophobic property are bounded by being mixed with each other to separately provide drainage paths and reaction gas diffusion paths (Japanese Patent Application Laid-Open No. H10-289723).
Further, there are also devised a method in which drainage grooves are provided in a surface of the catalyst layer, which is brought into contact with the GDL (Japanese Patent Application Laid-Open No. 2004-327358 and Japanese Patent Application Laid-Open No. 2005-38780), a method in which the drainage grooves are provided in surfaces of the GDL on current collector sides (Japanese Patent Application Laid-Open No. 2002-100372), and a method in which the GDL is provided with hydrophobic through holes and hydrophilic through holes (Japanese Patent Application Laid-Open No. 2003-151585).
However, with the conventional methods mentioned above, the following problems occur.
First, in a case of a structure described in Japanese Patent Application Laid-Open No. H10-289723, there is the following problem. It is expected that the drainage paths and the reaction gas diffusion paths in the GDL be formed by chance. Accordingly, a structure of the each path is not controlled, so there may be a case where the drainage paths and the reaction gas diffusion paths are longer than necessary. Alternatively, in a position where the both paths are connected midway to each other, drain water stops midway therethrough, so there may be a case where the gas diffusion path is clogged.
In a case of a structure described in Japanese Patent Application Laid-Open No. 2004-327358 and Japanese Patent Application Laid-Open No. 2005-38780, in which concave grooves for drainage are provided in the surface of the catalyst layer, which is brought into contact with the gas diffusion layer, there is a problem in that a catalyst-carrying amount is reduced by an amount corresponding to a volume of the grooves, so a reaction area of the catalyst is reduced, thereby reducing an output density of the fuel cell.
Further, with the structure described in Japanese Patent Application Laid-Open No. 2004-327358 and Japanese Patent Application Laid-Open No. 2005-38780, there is a problem in that, when the grooves are filled with the drain water, an area for suction to the catalyst layer is reduced.
Further, in a case of a structure described in Japanese Patent Application Laid-Open No. 2002-100372, in which the drainage grooves are provided in the surfaces of the GDL on the current collector sides, there is a problem in that, when the grooves are filled with the drain water, the area for suction to the catalyst layer is reduced.
Japanese Patent Application Laid-Open No. 2003-151585 describes a structure in which the gas diffusion layer is provided with the through holes and the through holes are completely isolated from each other into gas diffusion paths and water diffusion paths. As described in Example 2, in a case where a non-porous material such as metal is used for a diffusion layer substrate, it is difficult for a fuel gas to reach the catalyst brought into contact with the substrate, thereby causing the reaction area of the catalyst to be reduced. As a result, the flooding can be suppressed, but there is a problem in that, due to reaction gas supply rate control, a limit current density is reduced.
Further, in this structure, the through holes are completely isolated into the gas diffusion paths and the water diffusion paths. Accordingly, in a case of the fuel cell using the air breathing method in which a porous conductor made of foamed metal or the like is used in place of the separator, there is a problem in that a flooding suppression effect by the above-mentioned structure is obtained only in a restrictive manner.
A reason for this is that the conductive porous body has a porous structure in which all holes thereof are usually continuous with each other, so inside of the holes can be controlled only to be hydrophobic or hydrophilic, and although the gas diffusion through holes and the water diffusion through holes are isolated in the gas diffusion layer to any high degree, the both paths cross each other in the conductive porous body.
The related art technology involves the above-mentioned problems, and there has been a demand for effectively improving drainage performance of the gas diffusion electrode.